Add routine set_current_entry to unify setting of data->current_entry

This routine sets:

cache_inode_put(data->current_entry)
data->current_entry
data->current_filetype
data->current_stateid_valid = false;
data->current_ds->ops->put(data->current_ds);
data->current_ds = NULL;

It will optionally take an extra reference on the current_entry

It may be passed NULL for current_entry in which case it effectively
clears current_entry.

Signed-off-by: Frank S. Filz <ffilzlnx@mindspring.com>
Make SAVEFH and RESTOREFH work correctly with DS handles

Signed-off-by: Frank S. Filz <ffilzlnx@mindspring.com>